Is an oxygen concentrator the cheapest for home use (at night)? I can no longer afford the unit and have no insurance or Medicare.

Most home oxygen machines are new around $695 and you can find used ones at around $300. Since you already have a current oxygen company ask them if they are willing to supply you with a battery operated portable device.

Here is a resource for used machines: oxygenconcentratorstore/used-oxygen-concentrators/